Hi,
although this is an older thread and I assume you have resolved the problem it is a question that others might like to have answered.
I have a GA-790XTA-UD4 which has SATA3 and after re-configuring my rig to test it out I can say that yes, a SATA1 device, such as a DVD/CD drive will run and allow you to boot and install your OS (This was tested with Windows XP and Windows 7) when connected to a SATA3 port set to IDE mode.
However, I couldn't get Linux to see the drive until I re-set the system for AHCI Mode.
SATA3, like USB3 is backwards compatible.
Hope this helps